1. Field of the Invention
The subject invention relates to a control system for use with aircraft gas turbine engines, and more particularly to, a method of discriminating between spurious engine surges caused by, for example, disturbances to the inlet gas stream and genuine engine surges caused by, for example, the deterioration of the core engine or the malfunction of a critical engine component.
2. Background of the Related Art
The occurrence of a surge event in a gas turbine engine is often a precursor to a stall condition. Several techniques have been developed for detecting whether a surge event has occurred. A first technique compares engine control parameters with actual engine parameters. By example, the existence of a sustained difference between the demanded rate of change of engine speed and the actual rate of change of engine speed may indicate a surge condition. Another technique uses an engine signature to detect an engine surge, and relies primarily on a measurement of combustor burner pressure. In particular, this technique relies on sensing a transient spike in combustor burner pressure.
State-of-the-art adaptive digital control systems for helicopter gas turbine engines are configured to modify or adapt the baseline engine acceleration schedules following an engine surge event in an effort to compensate for and avoid future engine surges. However, these modifications are not permanently stored in computer memory because the surge event may have been spurious rather than genuine. If the surge event was spurious, permanently modifying the acceleration schedule would result in an reduction in engine performance over the present operating period, but more importantly would unnecessarily degrade engine performance during subsequent operating periods until maintenance is performed on the engine.
Therefore, in prior art control systems, which are not capable of making distinctions between spurious and true surge events, the adaptation of the acceleration schedule is stored in volatile computer memory (RAM) and the schedule is returned to the baseline values prior to the beginning of the next operating period.
Spurious engine surges can result from the ingestion of munitions gases, rocket exhaust gases or engine exhaust gases causing a distortion in the inlet air flow to the engine. Generally, spurious surge events are not repetitive and occur randomly. Genuine engine surges, on the other hand, often result from the deterioration of the core engine or malfunction of an engine component such as an inlet guide vane or bleed valve, and require repair or removal of the engine. Moreover, genuine surge events typically repeat within specific operating ranges, such as gas generator speed.
Often, after an initial engine surge event and subsequent modification of the baseline acceleration schedules of the engine, the remainder of the flight is without incident. However, since the acceleration schedule modifications are not stored in the permanent computer memory, if the initial surge event was indeed genuine, the engine will surge again on subsequent flights, and the engine performance will remain deteriorated. Under such circumstances, the engine would be removed from service.
U.S. Pat. No. 5,726,891 to Sisson et al. discloses a method of, and a system for, detecting an occurrence of a surge in a gas turbine engine. The method illustrated in Sisson includes steps, executed during consecutively occurring time periods, of: obtaining filtered derivatives of first and second engine operating characteristics; comparing the filtered derivatives of the first and the second engine operating characteristics to first and second threshold values, respectively; and incrementing a count only if both of the filtered derivatives exceed their respective threshold values. Otherwise, a next step decrements the count if one or both of the filtered derivatives do not exceed their respective threshold values. The method further includes a step of indicating a surge condition only if the count is equal to a predetermined value that is greater than unity. In a presently preferred embodiment of this invention the engine is a turbofan engine, the first engine operating characteristic is fan speed, and the second engine operating characteristic is exhaust gas temperature.
Although the Sisson et al method attempts to reduce the number of “false alarms” generated by transient conditions, it does so by evaluating the derivatives of two operating parameters over consecutively occurring time periods. A disadvantage of the Sisson et al. surge detection method is that it does not provide a method for distinguishing a spurious surge, which is a surge nonetheless, from a genuine surge. The Sisson et al. disclosure does not recognize that most genuine surge events often repeat at distinct operating regions and most spurious surge events occur randomly throughout the operating range of the engine.
It would be beneficial therefore, to provide a method of discriminating between spurious engine surges caused by disturbances to the inlet gas stream and genuine or true engine surges caused by the deterioration of the core engine or malfunction of critical engine components. Such a method would allow surge avoidance modifications to be permanently stored in non-volatile computer memory for subsequent application. The engine would remain in service until the next scheduled maintenance stop, thereby minimizing aircraft downtime.